Recently, size reduction and surface mounting have been progressed in the field of electronic equipment. For example, in a chip-type semiconductor ceramic electronic component, e.g., a PTC thermistor, an NTC thermistor and a varistor, it has been progressed to manufacture the electronic component in the form of a chip. As an example of such a semiconductor electronic component in the form of a chip, there is known a chip-type semiconductor ceramic electronic component disclosed in Patent Document 1. FIG. 6 is a schematic sectional view of a known chip-type semiconductor ceramic electronic component 11 disclosed in Patent Document 1. In the chip-type semiconductor ceramic electronic component 11, as illustrated in FIG. 6, first external electrode layers 13a and 13b made of, e.g., Ni having an ohmic property with respect to a ceramic body 12 are formed at opposite end portions of the ceramic body 12. Further, second external electrode layers 14a and 14b made of Ag having superior soldering performance and providing higher mounting performance with respect to a substrate are formed on surfaces of the first external electrode layers 13a and 13b, respectively.
The chip-type semiconductor ceramic electronic component 11 is manufactured as follows. First, on surfaces of a mother substrate from which the ceramic body 12 is to be obtained, the first external electrodes 13a and 13b made of, e.g., Ni having an ohmic property with respect to the ceramic body 12 are formed by an electroless plating method, for example. Then, both principal surfaces of the mother substrate are polished to remove the first external electrodes 13a and 13b, which are formed on both the principal surfaces, so that the first external electrodes 13a and 13b are formed on only side surfaces and end surfaces of the mother substrate. The mother substrate is cut to provide the ceramic body 12 in such a state that the first external electrodes 13a and 13b are formed only on the opposite end surfaces of the ceramic body 12. Thereafter, the second external electrodes 14a and 14b are formed on the first external electrodes 13a and 13b, respectively, by immersing the opposite end surfaces of the ceramic body 12 in an Ag bath. As a result, the second external electrodes 14a and 14b are formed in a state extending over part of the side surfaces of the ceramic body 12.
However, when, as disclosed in Patent Document 1, the second external electrodes 14a and 14b are formed by immersing, in the Ag bath, the opposite end surfaces of the ceramic body 12 on which first external electrodes 13a and 13b are formed, the second external electrodes 14a and 14b are generally baked onto the ceramic body 12 and the first external electrodes 13a and 13b by applying heat at temperature of about 600 to 800° C. after the immersion into the Ag bath. At that time, the heat applied to bake the second external electrodes 14a and 14b is conducted to the first external electrodes 13a and 13b as well. Accordingly, as illustrated in FIG. 7, the first external electrodes 13a and 13b having an ohmic property with respect to the ceramic body 12 may be caused to spread over the side surfaces of the ceramic body 12 depending on conditions of the heat treatment.
It has been proved that the above-mentioned phenomenon causes a variation in resistance values among individual chip-type semiconductor ceramic electronic components 11. Especially, a resistance value of the electronic component varies depending on an area of each of the first external electrodes 13a and 13b and a distance between the first external electrodes 13a and 13b in the case of the chip-type semiconductor ceramic electronic component 1 which has no internal electrodes inside the ceramic body 12, in particular, the distance between the first external electrodes 13a and 13b greatly affects the variation in resistance values among the chip-type semiconductor ceramic electronic components 1. For example, if diffusion of the first external electrodes 13a and 13b reaches the side surfaces of the ceramic body 12 and the first external electrodes 13a and 13b spread partly over the side surfaces of the ceramic body 12, resistance between outer peripheral edges of the first external electrodes 13a and 13b spreading over the side surfaces affects the resistance value of the chip-type semiconductor ceramic electronic component 11. Thus, a variation in distance between the first external electrodes 13a and 13b among the individual chip-type semiconductor ceramic electronic components 11 leads to the variation in resistance values among them and causes a serious problem.
Meanwhile, FIG. 8 illustrates a PTC ceramic electronic component disclosed in Patent Document 2. Patent Document 2 discloses a PTC ceramic electronic component 21 in which first external electrodes 23a and 23b each made of a Cr film are formed not covering corners of a ceramic body 22, and second external electrodes 24a and 24b are formed so as to extend over side surfaces of the ceramic body 22. Patent Document 2 further discloses that the first external electrodes 23a and 23b are formed by sputtering, for example, and the second external electrodes 24a and 24b are formed by baking a paste for the external electrodes.
Patent Document 1: Japanese Unexamined Patent Application Publication No. 5-29115
Patent Document 2: WO2007/118472
Even with the structure disclosed in Patent Document 2, however, when the second external electrodes are formed by using the paste for the external electrodes after forming the first external electrodes, heat is applied to the first external electrodes because the paste for the external electrodes is applied and baked by heat treatment.
For that reason, the first external electrodes may diffuse into the second external electrodes by application of heat, and the diffusion of the first external electrodes may spread up to portions of the second external electrodes, which extend over the side surfaces of the ceramic body, depending on conditions. This gives rise to a risk that the second external electrodes are given with the ohmic property and a variation in resistance values cannot be prevented satisfactorily.
Further, if the first external electrodes diffuse into the second external electrodes present on the end surfaces of the ceramic body, joining strength between the ceramic body and the first external electrodes reduces. Accordingly, the ohmic contact between the first external electrodes and the ceramic body is not obtained in some portions at a sufficient level, and a resistance change is increased in, e.g., a temperature cycle test that is conducted by repeatedly applying high and low temperatures (hereinafter referred to as “thermal shocks”). Thus, satisfactory reliability cannot be obtained in some cases.